The COOPER family I showed in CASS Co., IL is a different family than the one found in BUREAU & IROQUOIS Co.'s during the same time period. I just missed seeing the other listings when I looked the first time. So, yes, you can use it as a "process of elimination" so to speak. It looks like Wm. died somewhere in between BUREAU & IROQUOIS Co.
May I ask if you have subscription to Ancestry.com? If not, I can continue perusing it for the other family members.
Have you checked with BUREAU COUNTY GENEALOGICAL SOCIETY? They are a great group of folks and very helpful. http://www.rootsweb.ancestry.com/~ilbcgs/
Perhaps they could check the index for William for the dates between 1860-70? They have all of the Bureau Co. newspapers on microfilm, so if you can determine IF and WHEN he died in Bureau, they oould find the obit for you.
I am not familiar with IROQUOUIS Co, but I would go over to their MSG board and also check their genealogical society for more clues.
Best of Luck!
Family History Coordinator
Tampico Area Historical Societywww.tampicohistoricalsociety.com